In modern drilling operations, for example when drilling a wellbore in an oil or gas field, it is often desired to change the direction in the course of drilling. Generally one wishes to deviate the direction into which the drill bit at the lower end of a drill string progresses., away from the central longitudinal axis of the lower part of the drill string. Several drilling systems and methods have been developed for this purpose in the past.
U.S. Pat. No. 4,836,301 discloses a system and method for directional drilling. In the known system the drill bit is connected via a universal pivoting mechanism to the lower end of the drill string. The drill bit can be tilted so that the longitudinal axis of the drill bit can form a small deviation angle with the axis of the lower part of the drill string. The known system further comprises a steering means for rotating the drill bit in an orbital mode with respect to the lower part of the drill string. The steering means thereto comprises a flow deflector for providing hydrodynamical force in order to rotate the tilted drill bit azimuthally with respect to the lower part of the drill string as needed.
During normal operation of the known system, the drill string with the drill bit at its end is set to rotate, and the drill bit is tilted and counter-rotated in an orbital mode relative to the lower part of the drill string such that the axis of the drill bit remains geostationary.
The known system has the disadvantage that it requires large tilting forces on the bit, and that a complex but robust mechanism is needed for the universal pivoting mechanism in order to withstand the tilting and drilling forces at the same time.
Other systems known in the art are based on bending the lower part of the drill string above the drill bit, or on pushing the drill bit into the desired direction by applying side forces to the shaft of the drill bit.
These other systems also require complex and robust mechanisms in order to provide the large tilting forces to the bit.